Fix root directory detection

Do not falsely consider paths ending with a slash as root directories.
Fix conditional in `flycheck-find-file-in-tree`

The last branch needs a CONDITION.
Fix docstring
Fix `eval` cells in checker cell commands

Refactor the shell command creation to correctly substitute all special
argument cells, and quote their return values correctly.
Fix $HOME fallback when searching for files
Add tests for the checker API

Especially test the argument substitution and configuration file search.
Add test for `flycheck-find-file-in-tree`
Update tests for latest jshint release

Fix expected message, and remove broken test
Verify expected result types of `eval` cell

Also document the expected result types.
Add support for config files specified as single arguments

This change prepares for [#72] and the subsequent support for coding
standards in the PHP CodeSniffer checker, which does only accept options
and values given as single argument.